feat(i18n): add Spanish (es) locale (#798)

Adds Spanish as a UI display language, matching the existing
4-locale pattern (en, ja, zh-CN, zh-TW) with full key parity.

- app/src/i18n/locales/es/translation.json: 832 strings across 18
  namespaces, translated from the en master. Keys, {{interpolation}}
  placeholders, <code>/<path>/<link>/<strong> tags and _one/_other
  plurals preserved. Brand/model names (Whisper, Qwen3, CUDA, MCP…)
  left untranslated by design.
- app/src/i18n/index.ts: register `es` in SUPPORTED_LANGUAGES and
  resources; the language switcher and LanguageCode derive automatically.
- app/src/lib/utils/format.ts: wire the date-fns `es` locale for
  relative-date formatting.

Verified: key parity 832/832 (no missing/extra, placeholders & tags
intact), biome check clean, app+web typecheck pass, build:web succeeds.

Co-authored-by: Claude Opus 4.8 (1M context) <[email protected]>
Co-authored-by: Jamie Pine <[email protected]>

app/src/lib/utils/format.ts

@@ -1,5 +1,5 @@
 import { formatDistance } from 'date-fns';
-import { ja, zhCN, zhTW, fr } from 'date-fns/locale';
+import { es, fr, ja, zhCN, zhTW } from 'date-fns/locale';
 import i18n from '@/i18n';
 
 export function formatDuration(seconds: number): string {
@@ -10,6 +10,8 @@ export function formatDuration(seconds: number): string {
 
 function getDateLocale() {
   switch (i18n.language) {
+    case 'es':
+      return es;
     case 'ja':
       return ja;
     case 'zh-CN':